An immense amount of effort has been expended in understanding the photoperiodic control of reproduction in mammals. Despite this effort, an ignorance of great proportion still remains. Over three quarters of all mammals live in the tropics, yet the state of photoperiodic regulation remains tied exclusively to animal models drawn from the one quarter living in the temperate zone. We don't know if mammals have the capacity to react reproductively to the small variation in daylength they experience in the tropics. If they do have this capacity we don't know if they use it to regulate their reproduction seasonally. If this capacity is absent we don't know why in genetic and neuroendocrine terms it is absent. The present proposal speaks to all of these questions. Three species of small mammals will be trapped in a seasonally harsh environment in Venezuela where there is one hour of annual variation in daylength: a mouse-sized marsupial that is a rigid seasonal breeder; a rodent that breeds year around, and a rodent that is intermediate in its seasonality. Laboratory bred offspring of these animals will challenged to mature or short daylengths, both with and without added social influence. Where appropriate, critical daylength will be determined and compared with that existing when spermatogenesis and oogenesis start and stop seasonally in the wild. Thus we will learn whether or not these animals are reproductively photoresponsive and if some of them actually use this capacity. Species found to be unresponsive to photoperiod will be tested to determine why they are not responsive. Initial experiments will focus on the way they measure daylength, how they secrete melatonin, how they react to this hormone, and whether or not daylength influences the negative feedback sensitivity of LH secretion to gonadal steriods. Species found to be heterogenously responsive to photoperiod will be subjected to experiments designed to (a) see if reproductive photoresponsiveness can be reestablished in these animals via genetic selection; (b) explore quantitatively the genetic basis for the heterogeneity; and (c) determine how the loci of concern relate to neuroendocrine mechanisms. The results of all of this will be a new set of principles and animal models that are more truly representative of mammals as a whole.